Problem mixing * and _ for emphasis text

[torce]

Rendering *t_es_t* produces <p><em>t<em>es</em>t</em></p>\n.
The expected result is <p><em>t_es_t</em></p>\n, like github or kramdown does.
The same happens with the analogous case: _t*es*t_.

Script to reproduce the problem:

require 'redcarpet'
renderer = Redcarpet::Markdown.new(Redcarpet::Render::HTML)
renderer.render('*t_es_t*') # "<p><em>t<em>es</em>t</em></p>\n"